Nitrogen oxides (NO.sub.x) are well known pollutants which are generated during combustion, especially combustion carried out at high temperatures. It is known that the levels of NO.sub.x generated during a combustion reaction may be reduced if pure oxygen or oxygen-enriched air instead of air is used to carry out the combustion because the large volumes of nitrogen present in air are kept away from the combustion reaction and thus kept away from reacting with the oxidant to form NO.sub.x if high flame temperatures can be avoided. Two recent significant advances in this technical field which enable oxygen combustion to proceed without high flame temperatures are the aspirating burner, disclosed and claimed in U.S. Pat. No. 4,541,796 Anderson, and the stabilized jet burner, disclosed and claimed in U.S. Pat. No. 4,907,961 Anderson.
Combustion with oxygen or oxygen-enriched air generally produces a flame which is not readily visually observable. In some situations it is desirable to carry out combustion with an easily observable or luminous flame, especially where adjustment of the flame shape or direction is desired. A flame which is not easily observable is more difficult to adjust accurately.
It is known that the luminosity of a flame may be increased by carrying out the combustion in a manner wherein the fuel is not completely combusted. However, this mode of operation is undesirable both from a fuel consumption and an environmental perspective.
The problem of obtaining a luminous flame occurs primarily when natural gas is the fuel. The main component of natural gas is methane. It is necessary to break down the methane to form carbon since it is the presence of carbon particles which is the primary source of luminosity in a flame. Carbon particles may be obtained by carrying out combustion in a fuel-rich portion and an oxygen-rich portion. However, such procedure is difficult to control effectively and, moreover, significant levels of NO.sub.x are formed in the oxygen-rich region of the combustion.
Accordingly, it is an object of this invention to provide a combustion system wherein a luminous flame is generated while the level of pollutants such as NO.sub.x is maintained at an acceptable level.